1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor for a magnetic read head, more specifically, to the formation and material composition of its conductive lead layers.
2. Description of the Related Art
Magnetic read sensors that utilize the magnetoresistive effect for their operation fall broadly into two classes: those that use the anisotropic magnetoresistive effect (AMR) and those that use the giant magnetoresistive effect (GMR). AMR based sensors are simplest in structure, since they require a single magnetic layer whose resistance varies in proportion to the angle between its magnetization vector and the direction of electron flow (the sensing or bias current) through it. GMR based sensors are typically implemented in a more complex configuration, the spin-valve (SVMR). The spin-valve structure also involves a sensing current, but it has two separated magnetic layers, one whose magnetization direction can change (the free layer) and the other whose magnetization direction is fixed. The spin-valve's resistance is proportional to the angle between the magnetization of these two layers and the direction of the sensor current through them plays little role in the operation. Both forms of sensor require a method for providing a sensing current and that method typically involves the formation of thin, conducting lead layers on either side of the sensor. The spin-valve sensor also requires an additional magnetic layer, called the longitudinal bias layer, which maintains the free magnetic layer of the sensor in a stable orientation and a single magnetic domain state. In the SVMR sensor, the bias layer is typically formed of a hard magnetic material and is positioned on either side of the sensor element. Because of the location of the longitudinal magnetic bias layer, it has become accepted practice in the prior art to form the conductive lead layer directly over the magnetic bias layer. Given their relatively passive electromagnetic role in the sensor operation, the conductive lead layers need only satisfy certain basic, albeit stringent, material requirements. They must have a low sheet resistance, they must withstand the rigors of the harsh environment encountered during normal operation of the read head (eg. contact with the rapidly moving magnetic medium) and resist the equally harsh treatment associated with certain fabrication processes (eg. applications of corrosive chemicals) and they must not adversely affect the material properties of the magnetic bias layer on which they are formed.
The lead layers fabricated in accordance with the methods of the prior art have consisted mainly of layered structures such as Ta/Au/Ta, Cr/Ta/Cr, Ta/Mo/Ta, TiW/W/TiW. For example, McNeil (U.S. Pat. No. 5,479,696) provides a combination read/write magnetic head in which the conducting leads are a Ta/Au/Ta lamination.
Goubau et al. (U.S. Pat. No. 5,268,806) disclose a lead layer structure which comprises a thin film layer of body-centered-cubic (bcc) tantalum (alpha-phase Ta) which is separated from the sensor element by a thin film seed layer formed of material taken from the group consisting of TiW, TaW, Cr and W. The alpha-phase tantalum has a particularly low bulk resistivity of about 13 micro-ohm-cm at 300 K. The other layer provides a conforming substrate with similar atomic structure as well as corrosion and heat resistance.
Chen et al. (U.S. Pat. No. 5,491,600) disclose a multilayered conductive lead structure consisting of layers of conductive material alternating with layers of refractory metal, such as layers of gold/nickel alloy alternating with layers of tantalum. Gold is highly conductive, but its softness results in electromigration, smearing and nodule formation during sensor use. Tungsten has excellent conductivity and is harder than gold, but is subject to corrosion problems. Materials such as TiW/Ta have higher bulk resistivity and therefore require thick layers for adequately low sheet resistivity.
Pinarbasi (U.S. Pat. No. 5,883,764) discloses a method for forming very thin and highly conductive lead layers over the longitudinal bias layers of a spin-valve type read sensor. The lead layer structure comprises two adjacent seed layers of refractory metals deposited to modify the crystallographic texture of subsequent layers. A layer of highly conductive material is then deposited over said first and second seed layers. The structure finally provided by Pinarbasi comprises a CoPtCr hard bias layer over which is formed a conductive lead layer consisting of a Ta/Cr seed bilayer on which is then deposited a Ta lead layer.
As recording densities on magnetic media continue to increase, the associated read head sensors must become both narrower and thinner. Increasing the thinness of a sensor requires that both its longitudinal magnetic bias layer and the conductive lead layer formed over it become thinner. The formation of thinner longitudinal bias layers, in turn, requires new magnetic materials, structures and methods of formation. The formation of thinner lead layers requires conducting materials of extremely high bulk conductivity so that their sheet conductivity is correspondingly high as the material is formed in very thin layers. In addition, the materials comprising the conductive lead layers must retain the desirable properties of hardness, high melting point and corrosion resistance so as to survive the rigors of a harsh operating and fabricating environment. Most importantly, the formation of lead layers on longitudinal bias layers of new hard magnetic material requires careful attention to the physical consequences of crystallographical matching between the magnetic layer and the conducting layer and between the various material layers that comprise the conductive lead layer itself. It is towards these considerations that the objects of the present invention are addressed.